Session 5 ILO's - Mitosis and Meiosis Flashcards
Describe the relationship between DNA molecules, chromosomes and chromatids
DNA is stored as chromosomes within the cell’s nucleus. A chromosome is a single very long molecule of DNA containing a large number of genes. During the cell cycle chromosome are replicated so that have two copies of both the long- and short arms, giving the classic propeller shape.
A chromatid is one of the two identical halves of a chromosome that has been replicated in preparation for cell division. The two “sister” chromatids are joined at a constricted region of the chromosome called the centromere.
Outline and describe chromatid structure
- Produced from chromatin fibres
- A chromatid is one of the two identical halves of a chromosome that has been replicated in preparation for cell division. The two “sister” chromatids are joined at a constricted region of the chromosome called the centromere.
Outline and describe chromosome structure
- Chromosomes have both a long (q) and a short (p) arm
- Telomeres are repeat sequences found at the end of each chromosome/chromatid
- The sequence of the telomere is always TTAGGG
- Centromere is the sequence that links the sister chromatids together and consists of repetitive sequences
Understand the difference between a chromosome and a chromatid
A chromatid is one half of a replicated chromosome, whereas a chromosome consists of DNA wrapped around proteins in a highly organized manner.
Unpack the gene location: Xq26.2
- The gene is found on the X chromosome
- On the long Q arm
- 26.2 Centimorgans away from the centromere
What does the telomere do?
- Protects the ends of the chromosomes during replication, so that the entire number of genes on the chromosomes are copied, and no information is lost during replication
(In image, appear as fluorescent dots on either end of the chromosome/chromatids)
Appreciate how metaphase spreads can be used in chromosome analysis
If we open a cell during metaphase
(when chromosomes are condensed
for replication) we will see a jumble
of sizes and shapes
- It won’t be immediately apparent whether we have the correct complement of all the chromosomes or not
- However, you can sort out chromosomes and group them according to their size and shape
Appreciate how chromosome banding patterns can be used in karyotypes and ideograms
- We can further identify chromosomes by using G banding, used to stain chromosomes
- The chromosome undergoes light enzymatic digestion, followed by staining with the Giemsa stain
- This creates a characteristic banding pattern (black and white), which can be used to identify chromosomes, translocations duplications and loss of Y chromosomes
- Painting can also be used to identify chromosomes, where fluorescent markers label the different part of chromosome
Explain the difference between a gene and allele
- Gene is defined as a section of DNA that encodes for a certain trait. An allele is defined as a variant form of a gene.
- Genes on homologous chromosomes may contain different alleles
Explain how genetically identical daughter cells are created during mitosis
The cell copies - or ‘replicates’ - its chromosomes, and then splits the copied chromosomes equally to make sure that each daughter cell has a full set. The daughter cells are genetically identical because they each contain the same diploid chromosome complement as the original parent cell.
Explain how genetic variation is created during meiosis
Basically through recombination/crossing over and independent/random assortment:
- Recombination or crossing over occurs during prophase I. Homologous chromosomes – 1 inherited from each parent – pair along their lengths, gene by gene. Breaks occur along the chromosomes, and they rejoin, trading some of their genes. The chromosomes now have genes in a unique combination.
- Independent assortment is the process where the chromosomes move randomly to separate poles during meiosis. A gamete will end up with 23 chromosomes after meiosis, but independent assortment means that each gamete will have 1 of many different combinations of chromosomes.
This reshuffling of genes into unique combinations increases the genetic variation in a population and explains the variation we see between siblings with the same parents.
What is non dysjunction?
The unequal distribution of chromosomes, caused by failure to separate properly during meiosis.
Describe how non-dysfunction can lead to the symptoms of Down syndrome
- Caused by trisomy of chromosome 21
- Can result from non-disjunction during meiosis
- Chromosome 21 has failed to separate properly, giving rise to 1 oocyte without a copy of chromosome 21, and one which has 2, instead of a single copy
- When this oocyte fuses with a sperm, it will produce an offspring with 3 copies, the trisomy of chromosome 21, which can lead to symptoms of Down syndrome
What is aneuploidy?
The condition of having an abnormal number of chromosomes in a haploid organism
What is aneuploidy caused by?
Aneuploidy can be caused by non-disjunction during mitosis or meiosis. A failure to
separate pairs of chromosomes lined up at the metaphase plate can produce daughter
cells with abnormal chromosome numbers.
In some cases, you could have 1 copy of a chromosome, instead of 2, and in others, 3 copies instead of 1
Explain how mitotic nondisjunction can lead to trisomy and mosaicism and how meiotic nondisjunction leads to trisomy and monosomy
Aneuploidy can be caused by non-disjunction during mitosis or meiosis. A failure to
separate pairs of chromosomes lined up at the metaphase plate can produce daughter
cells with abnormal chromosome numbers.
In some cases, you could have 1 copy of a chromosome, instead of 2 (monosomy) , and in others, 3 copies instead of 1 (trisomy)
Describe the process of meiosis in males (spermatogenesis)
- Primary cell = spermatogonium
- This matures into primary spermatocyte
- During meiosis, 4 spermatids, which are haploids, are produced
- These mature into mature sperm, which are haploid
Describe the process of meiosis in females (oogenesis)
- Primary cell = oogonium
- This matures into primary oocyte
- During meiosis, 1 ooycte and 3 polar bodies are produced. The polar bodies are not used for reproduction
- The oocyte then matures to form a mature ovum
Compare and contrast spermatogenesis and oogenesis
- The length of oogenesis is 12-50 years
- The length of spermatogenesis is approx 60 days
-1 oocyte (2n) gives to rise to 1 egg (n) + 3 polar bodies
-1 spermatocyte (2n) gives rise to 4 sperm (n)
Compare and contrast mitosis and meiosis
What is meant by the term ‘diploid’?
Carries 2 copies of each chromosome - 1 is maternal, 1 is paternal
(Human somatic cells are diploid)
What is meant by the term ‘haploid’?
The presence of a single set of chromosomes in an organism’s cells. In humans, only the egg and sperm cells are haploid
Describe what causes Down syndrome
- Trisomy of chromosome 21
- The down syndrome patient has 3 copies of chromosome 21 instead of 2
- Caused by non-disjunction during meiosis
Give 3 features of Down syndrome
- Delayed growth
- Characteristic facial features
- Mild intellectual disability
Learn the positions of the centromere
- The positions of the centromere differs between different types of chromosome, and it can be used to classify chromosomes into different groups:
- Metacentric - centromere found in centre of chromosome
- Submetacentric - centromere found nearer to the top of the centre
- Acrocentric - centromere found close to the top of the chromosome
- Telocentric - centromere found on the very end of the chromosome
Outline how to group chromosomes
- Chromosomes are grouped according to their size and shape
- In humans, we have 7 groups based on the size, position of centromere and local differences
- We have groups of A-G and also have the sex chromosomes
- X is a C group member, and Y is a G group member
Understand the term allele
Alternative forms of the same gene that are found at the same place on a chromosome
- Sister chromatids have identical DNA and thus the same alleles
Understand the term chromosome and chromatid
- One chromosome = one molecule of DNA, which can code for hundreds, if not thousands of different genes
- They are both ‘one single chromosome’ but the replicated chromosomes now contains 2 DNA molecules
- When the chromosome is replicated, one chromosome is two DNA molecules
Understand the terms sister and non-sister chromatids
- Sister chromatids are the duplicated chromosome itself, they contain the exact same DNA and alleles.
- Non-sister chromatids are the chromatids of the homologous chromosomes
- They have the same genes, but may have different alleles (variants of a gene) – derived from the
maternal and paternal genomes
Understand the term ‘homologous chromosome’
- A set of one maternal and one paternal chromosome that pair up with each other inside a cell during fertilization.
- Homologous chromosomes have the same genes
Describe the stages of mitosis
Prophase:
- Nuclear envelope breaks down.
- Nucleolous disappears.
- Spindle fibres appear and chromosomes
condense
Prometaphase / Metaphase:
- Spindle fibres attach to chromosomes.
- Chromosomes align and condense at plate (centre of cell)
Anaphase:
- Centromeres divide.
- Sister chromatids move to opposite poles
Telophase:
- Nuclear membrane reforms.
- Chromosome decondense and spindle
fibres disappear
Cytokinesis:
- Cytoplasm divides and two daughter cells with identical genetic
information are produced
Meiosis summary steps
Describe Prophase 1 of meiosis
Prophase 1:
- Nuclear membrane dissolves
- Chromosomes condense
- Prophase can last up to several days in mammals, because recombination takes time
Recombination:
-During prophase I homologous chromosomes pair up to form structures called bivalents, then crossing over can occur between the 2 copies of the chromosome - the maternal and paternal copy
- The different sister chromatids cross over at the chiasmata
- This leads to the formation of recombinant chromosomes
Describe recombination of Prophase 1
-During prophase I homologous chromosomes pair up to form structures called bivalents, then crossing over can occur between the 2 copies of the chromosome - the maternal and paternal copy
- The different sister chromatids cross over at the chiasmata
- This leads to the formation of recombinant chromosomes - daughter chromosomes, which are genetically different from those at the parental chromosomes
Describe Metaphase 1 of meiosis
- Spindle apparatus forms and the chromosomes line up at the centre of the cell (at the metaphase plate)
What are the cells produced by meiosis like?
- Haploid (Have half the number or chromosomes)
How can chromosomes be analysed?
Metaphase spread:
- Stained metaphase chromosomes
- Actively dividing cells are needed
What is mosaicism?
- Mosaicism is the presence of 2/more cell lines in an individual
Mosaicism occurs when a person has two or more genetically different sets of cells in his or her body. If those abnormal cells begin to outnumber the normal cells, it can lead to disease that can be traced from the cellular level to affected tissue, like skin, the brain, or other organs.
- It can occur throughout the body or be tissue limited
- Non-disjunction can cause mosaicism
Recognise each mitotic (and meiotic) phase on photographs or diagrams
Mitosis photograph
Recognise each mitotic (and meiotic) phase on photographs or diagrams
Meiosis photographs
Recognise each mitotic (and meiotic) phase on photographs or diagrams
Mitosis diagram
Recognise each mitotic (and meiotic) phase on photographs or diagrams
Meiosis diagram
